(1) Field of the Invention
The present invention relates to a noise reduction apparatus for reducing noises of a television picture signal, particularly for effectively reducing only noise components thereof without any deterioration of picture quality in company therewith conventionally.
(2) Description of the Prior Art
Generally, noise components of a television picture signal are contained in a higher frequency range thereof, so that those noise components can be reduced by using a low pass filter as one of the most simple reduction method. However, in a situation where only the low pass filter is employed in a simple manner, high frequency components of the picture signal are reduced together therewith, and, as a result, the picture quality is extremely deteriorated.
In contrast therewith, the television picture signal has a a strong interframe correlation as one of distinct features thereof, so that random noise components having not such interframe correlations at all can be effectively reduced by the separation based on the interframe correlation.
On the other hand, with the recent progress of the coded television picture signal transmission, a picture signal interframe coding system for reducing an amount of coded signals to be transmitted by utilizing the above mentioned interframe correlation of the television picture signal is being developed. However, a troublesome problem regarding a moving picture which is usual for the television is caused by the weakened interframe correlation based on the motion of the picture, and hence has been left unsolved hitherto together with the above mentioned present problem regarding the interframe noise reduction of the moving picture signal. The present inventors disclosed "A Motion-Compensated Interframe Coding System" according to U.S. Pat. No. 4,307,420 as one of solutions for the above mentioned unsolved problems, in which system a motion-compensated interframe noise reduction accompanied with the motion-compensated interframe coding of the picture signal was proposed.
In a motion-compensated interframe noise reduction apparatus of the above mentioned kind, in which an input picture signal is positively added with a one-frame delayed picture signal thereof through a frame memory, the resultant picture signal being normalized, for intensifying picture signal components based on the interframe correlation thereof, as well as for averaging random noises having no interframe correlation, so as to reduce those noises in an output picture signal, the reduction of varying signal components losing the interframe correlation due to the motion of the picture together with those noises can be prevented by positively adding a motion-compensated predicted picture signal in which the lost interframe correlation has been restored by compensating those varying signal components in the moving portion of the picture in response to a result of detection of the motion, so that those noises can be separately reduced even in the moving portion of the picture.
In this motion-compensated interframe noise reduction apparatus, the above mentioned motion-compensated predicted picture signal is added in each block of picture elements having an appropriate region by switching over from an ordinary predicted picture signal in response to the result of detection of the motion in the above each region. However, the motion of the picture is detected under the descrimination of an amount of interframe difference of the picture signal, so that, when the amount of the interframe difference is increased by the noise, the motion-compensated predicted picture signal is positively added to stationary portions of the picture signal and hence the picture quality is deteriorated rather than improved.
On the other hand, in the interframe noise reduction apparatus of the above mentioned kind, the noise reduction rate is generally lowered in moving portions of the picture, because, in a situation where the noise reduction rate is not lowered, the above mentioned secondary deterioration of the picture quality is caused by the noise reduction. One of solutions for this problem of the secondary deterioration of picture quality is the above mentioned motion-compensated interframe noise reduction. However, under certain circumstances, the sufficient noise reduction cannot be attained in moving portions of the picture. In these circumstances, the noise reduction cannot help being effected by an inframe signal treatment instead of the above mentioned interframe signal treatment. The principle of this inframe noise reduction is in a word that the noise is reduced through a spatial low pass filter. However, in a situation where only the low pass filter is employed in a simple manner, the secondary deterioration based on the reduction of higher frequency components of the picture signal is caused as mentioned earlier.
In addition, it is further defective that the signal to noise ratio is not so increased subjectively by reducing noise components in a frequency range higher than 1 to 2 MHz, and hence the pass band of the low pass filter used for the noise reduction cannot help being considerably narrowed. Consequently, the picture quality obtained by simply employing the low pass filter cannot help being extremely deteriorated.
In regard to the above problem, several kinds of inframe noise reduction apparatus for preventing the above mentioned secondary deterioration of picture quality which is caused by the employment of the filter have been proposed. It is an example thereof to employ a well-known Wiener filter by which the noise reduction is effected by utilizing the different of the statistical performance between the picture signal components and the noise components of the television picture signal. However, these conventionally proposed noise reduction apparatus of this kind have a common defect that the circuit configuration and the process of signal treatment are considerably complicated, and hence it is so difficult to treat the television picture signal in real time.
Apart from the above proposed apparatus, another kind of conventional inframe noise reduction apparatus for utilizing the visual property of the television picture has been proposed. Generally, an extreme gradation, namely, lack of sharpness of the television picture is visually perceived due to the gradation caused in edge portions thereof, so that the employment of the low pass filter having a narrow pass band regarding flat portions of the television picture can reduce the visual gradation caused on the whole of the picture by virtue of the lowered visual sensitivity regarding the edge portions thereof less than regarding the flat portions, although more noise components are left in those edge portions in comparison with the flat portions. However, this preferable noise reduction apparatus has also another defect such as the employed low pass filter necessitates to use non-linear circuit components and to complicate the conventional circuit configuration required for obtaining the above mentioned preferable performance.